Typically a transmitter or receiver component are mounted in a metal panel. A receptacle which may extend outwardly through an aperture in the panel is provided for receiving an optical fibre, and a package extending behind the panel houses the transmitter or the receiver. In known optical sub assemblies the receptacle houses a ceramic ferrule containing an optical fibre stub and a package has an aperture for receiving the ceramic ferrule. The receptacle and the package may be aligned to obtain an optimum coupling efficiency between the fibre stub and the optical device. The receptacle is secured to the package by soldering, welding or brazing. In order to maintain dimensional stability the receptacle and the package are formed from a stable material such as stainless steel.
In order to prevent electromagnetic emissions, the panel is earthed (grounded) through the mains, which results in the package housing the optical device being earthed through the mains. However, the transmitter and receiver operate at very high frequencies (GHz) and they are supplied with electrical power at low voltages from stabilised power supplies. These power supplies are generally earthed independently of the casing, which means that the earth on the housing of the optical devices is not the same earth as the earth on the electronic circuit boards upon which the optical devices are mounted.
High frequency operation results in a radio frequency emission problem. One way in which the emission problem could be reduced would be to provide the same earth for the power supply and the mounting panel. However this is counter to the user requirement for freedom to have a different earth for the power supply and the mounting panel, so the problem to be solved is to provide an optical sub assembly in which the receptacle and package may be electrically isolated from one another.
This problem is not as straightforward to solve as it might initially seem, due to the dimensional stability required for the receptacle. Furthermore, for a sub assembly for an optical transmitter there may be a requirement to align a ceramic ferrule housed in the receptacle with the optical transmitter housed in the package prior to securing the receptacle to the package. The securing process must maintain an optimum alignment within typically 0.1-1 μm (micron).